In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station node, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) (also known as the Long Term Evolution (LTE) radio access) and the Evolved Packet Core (EPC) (also known as System Architecture Evolution (SAE) core network). E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to radio network controller (RNC) nodes. In general, in E-UTRAN/LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and the core network. As such, the radio access network (RAN) of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
In a homogeneous deployment with a single cell layer, the transceiver devices that are sensitive to interference are usually also the ones that cause high interference to transceiver devices in adjacent cells. In the uplink (UL) the reason for this concurrence is the following: a sensitive transceiver device is one that has high pathloss to the serving base station, and therefore the power received by the serving base station is low, in particular if the transceiver device reaches its power limit. A transceiver device with high pathloss is typically at the cell border (commonly called cell-edge transceiver device), which is where it is also closest to adjacent cells (and adjacent base stations). For these adjacent cells the transceiver device (especially when operating at its power limit) is typically a strong interferer.
The growing demands on mobile networks to support data applications at higher throughputs and spectral efficiencies has driven the need to develop Orthogonal Frequency Division Multiplexing (OFDM)-based 4th generation (4G) networks including for 3GPP Long Term Evolution (LTE). A key objective with respect to deployment of OFDM 4G networks is to utilize a frequency re-use of one (denoted by N=1), or as close to N=1 re-use as is practical. A frequency re-use of N=1 implies that the base stations in cells transmit on all available time-frequency resources blocks (RBs) simultaneously. Due to transmit power limitations in mobile terminals, the need for higher throughputs in 4G networks, especially near the cell edge, combined with the constraint on the uplink link budget will necessitate the need for smaller cell sizes than is typically deployed for present 2nd generation (2G) and 3rd generation (3G) cellular systems.
The use of smaller cells sizes can be deployed in a traditional homogenous cell splitting approach or in a more ad hoc heterogeneous approach in which pico cells or relay nodes are overlaid on an existing macro cellular network. For both a homogeneous and heterogeneous approach, the resulting interference limited system for N=1 deployment will not achieve the full potential capacity that the LTE standard can support without the implementation at the base station and mobile terminal of one or more viable interference mitigation and or cancellation techniques.
Interference cancellation and mitigation techniques have been investigated and deployed with varying degrees of success in terrestrial mobile networks for over twenty years. Traditional approaches to interference mitigation between transmitted signals have focused on either ensuring orthogonality between transmitted signals in time, frequency as well as spatially or by actively removing and cancelling interfering signals from the desired signal if orthogonality between the desired signal and potential interferers cannot be achieved. In early 2G cellular systems such orthogonality was achieved primarily through static pre-planned allocations of radio resources.
3G systems introduced interference cancellation techniques based mostly on a combination of blind information gathering at a base station such as spectrum usage monitoring and coarse exchange of interference indicators such as the Rise over Thermal (RoT) indicator employed in the 3GPP2 1xEV-DO standard. Typically interfering signals have been estimated using blind detection and their estimates subtracted from the desired signals.
From a link perspective the downlink (DL) allows for a more tractable analysis since if the desired mobile terminal location is known, the distances to all potential interfering base stations can be easily determined based on the network geometry and hence a probabilistic based estimate of the signal-to-interference-plus-noise (SINR) can be calculated based on the channel fading conditions for the desired signal and the interfering signals. In addition to additive white Gaussian noise (AWGN), both the desired signal and interfering signals will experience shadowing which typically is log-normally distributed.
Analysis of the uplink (UL) interference requires knowledge of not only the location of the desired mobile terminal under consideration, but also the relative locations of all potential interfering mobile terminals, for which both the locations of the interfering terminals, the number of potential terminals as well as their spatial velocity will be random variables.
In cellular networks it is a well known problem that, in medium to heavy loading, the network becomes interference limited which can result in negative signal-to-interference-plus-noise (SINR) ratios, particularly for cell edge users.
The challenge with deploying a static N=1 frequency re-use OFDM system in an interference limited environment is that for a fully loaded deployment, significant regions of coverage will experience negative SINR levels resulting in gaps in the deployed coverage, irrespective of the inter-cell distance. In an interference limited system it is not uncommon for on the order of 15% of users to experience negative SINR, with some users experiencing negative SINR levels of −10 to −15 dB. It should be noted that in a fully loaded interference limited cellular deployment the severity of the SINR degradation will be highly dependent on the average path loss exponent. For a cellular deployment with a fixed inter-cell distance, high path loss propagation environments with path loss exponents up to a 5th or 6th order will experience less overall interference than deployments with lower path loss exponents, since potential interfering signals from neighboring cells will be more greatly attenuated in the former case. Even though there will be significant SINR variation depending on the propagation environment, in order to robustly deploy an LTE OFDM system one will have to mitigate the inevitable negative SINR coverage regions that will exist.
Fractional frequency re-use (FFR) is one approach that can be statically or adaptively employed in heterogeneous cellular network deployments to improve the overall geometry and SINR levels, particularly for cell edge users. However this gain in SINR is typically at a cost of a decrease in overall aggregate cell throughput and spectral efficiency. For example, overall throughput is reduced to about 70% of an N=1 deployment if N=⅓ FFR is employed.
Use of pico-cell or relay node overlays on existing macro cellular deployments can also be employed to improve cell coverage as well as increase cell edge or overall cell throughput. However macro/pico-cell heterogeneous deployments suffer from a number of potential problems. In LTE Release 8, cell selection between macro-cell base stations and pico-cell base stations will typically be based on use of reference symbol received power (RSRP). With such an approach, macro-cell UEs near the macro cell edge will typically be transmitting with high power and can cause a high level of interference to nearby pico-cell base stations. On the downlink (DL) if the UE has open access to either the macro or pico base stations, the UE can connect to the best link. However, at the border between the macro and pico cells the signal-to-interference (SIR) level can be low. In such a situation, inter-cell interference-coordination approaches can be beneficial. However if access to the pico or femto-cells of the heterogeneous network is restricted or closed (e.g., closed subscriber groups or CSGs), the femto-cell base stations can cause a high level of interference to nearby macro UEs that cannot handover to the femto base stations.
A second possible approach for cell selection between macro and pico base stations is to employ a path gain approach which is optimal for load balancing. With such an approach the UL signal strength will generally be robust, however the SIR at the macro-pico cell borders may be low. With respect to the DL, high interference may be experienced by the pico UEs from the macro base station transmissions for both the control and data channels. Furthermore, for a CSG scenario, macro UEs close to the pico base station can only connect to the macro base station and will be a source of high interference to the pico base station for UL transmissions.